configure.ac: fix build with --enable-bufferevents
When building with bufferevents enabled, configure.ac throws an
error if "$ac_cv_header_event2_bufferevent_ssl_h" is not set to
"yes". However, nowhere was AC_CHECK_HEADERS(event2/bufferevent_ssl.h)
done. This commit adds the check.
Anthony G. Basile%!s(int64=10) %!d(string=hai) anos